Earliest Galaxies Helped Lift Universe's Cosmic Fog

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Researchers are building a better picture of one of the most
important changes in the early universe a process that lifted the
cosmic fog from the dark, early universe.

A new study of observations by the Hubble
Space Telescope has revealed that the earliest known
galaxies, present some 800 million years after the Big Bang 13.7
billion years ago, may have emitted enough ultraviolet light to
strip electrons from (or ionize) the hydrogen gas between
galaxies.

For years, researchers have believed that as these galaxies
coalesced into larger and larger structures. Their subsequent
ultraviolet glows ionized hydrogen in bubbles around them, which
grew and eventually overlapped until essentially all the
intergalactic medium was ionized and transparent.

But researchers were less sure about the timing.

"That has been the outstanding question," Brant Robertson of the
California Institute of Technology in Pasadena, Calif., told
SPACE.com. "At what point in time do galaxies produce enough of
these ionizing photons to keep the electrons and protons in the
universe unbound?"

Although the universe actually began in an ionized state
following the
Big Bang, this first period of ionization lasted for only
about 300,000 years, until electrons and protons cooled enough to
form hydrogen atoms. (This hydrogen formation generated photons
that we know today as the cosmic microwave background.)

What followed was a period known as the cosmic Dark Ages, during
which the universe's vast clouds of cloud hydrogen slowly
condensed to form the first galaxies. From studies of neutral
hydrogen detected in the light from distant quasars, astronomers
knew that re-ionization must have been complete by about a
billion years after the Big Bang.

"The thing that's changed recently is the installation of the new
camera on Hubble," Robertson said.

Hubble's Wide Field Camera 3, in operation since May 2009, has
allowed astronomers to pinpoint the infrared signatures of more
than 50 galaxies that date to 800 million years after the Big
Bang.

The research is detailed in the Nov. 4 issue of the journal
Nature.

Explaining the cosmic fog's lifting

By determining whether these
ancient galaxies released sufficient ultraviolet light to
re-ionize the universe's intergalactic hydrogen, researchers
could rule out other possible explanations for re-ionization,
such as radiation from matter falling into massive galactic black
holes, or the annihilation of hypothetical dark matter particles.

In their new analysis, Robertson and his colleagues first used
the recent Hubble data to estimate the total number of ancient
galaxies. Then they took information about the color of light
from those galaxies to estimate the sizes and intensities of the
stars contained in them, which yielded the number of ultraviolet
photons generated by each galaxy.

Finally, based on data for closer galaxies, the researchers
estimated that some 10 to 20 percent of those ultraviolet photons
would have made it out of their galaxies to re-ionize
intergalactic hydrogen.

The results suggest there were likely enough ultraviolet photons
to re-ionize the universe in the first 800 million years of
cosmic time, although uncertainties remain.

Robertson said one of the major uncertainties in their
calculation was the number of early galaxies, which currently
depends on Hubble's surveys of a small section of sky.

But as Hubble collects data over a wider portion of the sky, that
uncertainty will decrease.

"It's a very exciting time," Robertson said. "In the next couple
of years we'll be able to know if there are enough galaxies to
re-ionize the universe."